There's a great article on Projects@Work about Harvard Pilgrim Health Care's (HPHC) PMO, examining the elements that have made it succesful. It's rare you hear about a successful PMO these days, and when you do, it's usually a result of the factors listed here.
Key lessons (all of which I've long endorsed):
1) Look outward to the overall business and the customer, not just IT.
2) Keep it simple. A PMO doesn't have to be complex; it just needs to support project managers, and engage them in continuously improving the overall process.
3) Keep it integrated. An enterprise PMO, outside of IT, is a vital part of ensuring integration. An additional IT PMO is fine, as long as it's integrated with the whole.
This is in line with my statements that a PMO should really be called an IMO (Integration Management Office).
Here's an excerpt that explains the way the Enterprise PMO and an IT PMO work together at HPHC :
HPHC has two formal PMOs; one at the enterprise level, the other within the IT Division, managed by outside vendor Perot Systems. The two units have co-existed since 1999 and have no trouble defining their roles and relationships in support of business plan delivery. According to Ron Hill, Perot Systems Client Executive to HPHC, the pathway for success is “born from the teamwork of the EPMO and the IT PMO.” Hill believes the strength of the relationship between the two units is visible through the communication between the EPMO, the IT PMO, and the business users. “Everyone must use the same methodology, vernacular, and project tools to reduce the risk of miscommunication.”And here's the full article...